When a surface is exposed to stress, the quality of that surface often declines. The durability of a surface is its ability to resist such a decline in quality when exposed to stress. The quality of a surface may be, for example, its appearance, strength, integrity, other property or properties, or a combination thereof. Stress on a surface may arise, for example, from mechanical forces, chemicals, radiation, erosion, wear, other form or forms of stress, or a combination thereof.
In some situations, when an item is put into use, at least one of its surfaces is exposed to stress during the conditions of normal use, and the quality of that surface declines relatively slowly. It is often desired to find a way to predict, without actually exposing a given surface to normal use for a long period of time, the extent and type of degradation in quality that that surface will experience if it were to be exposed to normal use for a long period of time (i.e., two weeks or more; or 30 days or more; or 60 days or more). One way to make such a prediction accurately is to expose a surface to conditions of stress that cause the quality of the surface to degrade relatively quickly, in a way that mimics the degradation caused by the conditions of normal use. In some situations, use includes various causes of stress, including, for example service and cleaning. Service includes the various conditions other than cleaning operations to which the surface is exposed.
For example, U.S. Pat. No. 5,522,251 describes a device in which a sample of carpet or carpet tile on a rotating table is exposed to the following: a wear-causing device that rolls caster wheels over the carpet sample, a shampoo head, a dryer, a vacuum extraction head, and a carpet sweeper. It is desired to provide an apparatus that exposes surfaces to service-type conditions (i.e., conditions that reproduce normal service conditions or that mimic the effects of normal service conditions) as well as to cleaning-type conditions (i.e., conditions that reproduce normal cleaning conditions or that mimic the effects of normal cleaning conditions). In some cases, it is further desired to provide an apparatus that has one or more of the following advantages: the exposure to service-type conditions and the exposure to cleaning-type conditions can be controlled independently of each other; the apparatus can be controlled with a computer program; the sample of surface is moved linearly; or any combination thereof.